GCC with Cortex-M4 hard float link error "X uses VFP register arguments, Y does not"

[Johannes Bauer]

Hi list,

I have a problem with gcc-4.7.1 (and g++) for ARM, specifically the
Cortex M4 (Thumb2 variant). I've compiled it with newlib and multilib
support and am now trying to get hard float running, with no success so
far. Showing that multilib is enabled:

$ arm-none-eabi-g++ -print-multi-lib
.;
thumb;@mthumb
fpu;@mfloat-abi=hard

I compile with

arm-none-eabi-gcc -Wall -O2 -g -mcpu=cortex-m4 -mfloat-abi=hard -mthumb
-std=c99 -c -o foo.o foo.c

Compiling the modules works fine and hard float instructions are generated:

0000013c <floattest(float)>:
 13c:   ed9f 7b08       vldr    d7, [pc, #32]   ; 160
<floattest(float)+0x24>
 140:   eeb7 0ac0       vcvt.f64.f32    d0, s0
 144:   ee20 0b07       vmul.f64        d0, d0, d7
 148:   ed9f 7b07       vldr    d7, [pc, #28]   ; 168
<floattest(float)+0x2c>
 14c:   eeb7 0bc0       vcvt.f32.f64    s0, d0
 150:   eeb7 0ac0       vcvt.f64.f32    d0, s0
 154:   ee30 0b07       vadd.f64        d0, d0, d7
 158:   eeb7 0bc0       vcvt.f32.f64    s0, d0
 15c:   4770            bx      lr
 15e:   bf00            nop
 160:   e631f8a1        .word   0xe631f8a1
 164:   40c81cd6        .word   0x40c81cd6
 168:   692b3cc5        .word   0x692b3cc5
 16c:   3f5437c5        .word   0x3f5437c5

When it comes to linking, however, this is a different story. For each
module "m" and the final ELF target "t" I get a message:

[...]/bin/arm/lib/gcc/arm-none-eabi/4.7.1/../../../../arm-none-eabi/bin/ld:
error: m.o uses VFP register arguments, t does not
[...]/bin/arm/lib/gcc/arm-none-eabi/4.7.1/../../../../arm-none-eabi/bin/ld:
failed to merge target specific data of file m.o

I'm guessing it's trying to link newlib stuff in from the libcrt* which
might not be compiled with hard float, can that be correct? Then why
would gcc's multilib show the correct strings? Could somebody please
explain to me what I'm doing wrong?

Thank you very much,
Joe